This invention relates to a high speed stacking device for delivering flexible sheets, for example, sheets of paper from a flowline to a stacking station.
This invention has particular utility in apparatus adapted for high speed handling of banknotes, for example, in a banknote-condition verifying machine in which mixed new and used banknotes are fed from a stack, examined by one or more analysing means, and thereafter routed to various stack forming devices in accordance with their condition. Such machines are required to handle large quantities of banknotes and it is therefore necessary to feed the banknotes at a high speed, for example, in the order of 20-25 banknotes per second. To permit the various routing devices to act individually on the banknotes adequate spaces must be formed between adjacent banknotes along the flowline, inevitably leading to a requirement for a high linear velocity. Special problems are encountered at each of the stacking stations because it is necessary to decelerate the banknotes up to a stationary support surface in a controlled progressive manner to ensure that uniform and aligned stacks are formed. Further problems arise when it becomes necessary to handle used banknotes because of their indeterminate condition, for example, they may be torn or partially folded, and furthermore, the structure of the paper may have weakened or been degraded through use, and this may impair the efficiency of a vacuum feed. In such circumstances it is possible for a bad banknote to deviate from its intended path or to skew at a stacking station with a consequential risk of jamming the flowline and arresting operation of the machine.
To facilitate the formation of uniform and aligned stacks at a stacking station it is known to provide a wheel having a sheet-receiving slots presenting openings substantially tangential to the wheel at points around the wheel periphery, the slots defining spiral paths for sheets of paper fed into the openings as the wheel rotates. The wheel is driven at a peripheral speed lower than that of the flow-line and in a direction such that the openings trail in the direction of rotation. Examples of such wheels are shown in British Pat. No. 988,382 in which the wheel is of a type formed by providing spirally extending slots in a disc, and British Pat. No. 852,005, in which the wheel is of a type having a hub to which suitably directed tines are attached to form the sheet-receiving slots and to define the spiral paths. Such devices are so disposed that each of the sheet-receiving slots is in turn presented tangentially to the flow-line so that a free leading end of a sheet fed along the flow-line, at a linear speed greater than that of the wheel periphery, enters the slot entrance presented thereto and the sheet is propelled into the slot. The sheet is decelerated as it progesses along the spiral path. The sheets in the slots are successively stripped off during continued revolution of the wheel to form a stack on a sheet support surface.
In the known forms of stacking apparatus employing slotted wheels, the sheets to be stacked are advanced between belts or rollers, for example, towards and into the slots. As explained above, sheets such as used banknotes are frequently in poor condition and their flimsiness and possible folding makes it difficult to feed them in a uniform manner into a slotted wheel to take advantage of the potential of the slotted wheel for uniform stacking.